It is already known for many years, to prepare composites having a metal, or optionally, plastic matrix, by using as reinforcement material, fibers or whiskers in ceramic material such as silicon carbide.
These composites can be prepared according to different technologies, for instance by mixing the fibrous material with metal powders and by subjecting the whole to a thermal treatment, generally under pressure, to melt the metal matrix. Alternatively, the ceramic fibrous material can be used for preparing porous preforms to be infiltrated by the molten metal according to known technologies.
Methods for preparing preforms based on fibers or whiskers are described in the literature such as, for instance, in U.S. Pat. Nos. 4,587,774 and 4,740,428.
According to these methods, preforms can be prepared suitable for infiltration by a molten metal, either by letting the whiskers grow directly in a porous mass or by dispersing the whiskers themselves in polar liquids such as alcohols, water, ketones and the like. Such dispersions can be additioned with a polymeric binding agent soluble in the aforesaid polar liquids and then poured into molds having the desired shape and turned into porous preforms after having removed, by heating or another technique, the solvent and the possible polymeric binder.
In both cases preforms are obtained, having a high porosity, consisting of an agglomerate of short fibers and/or whiskers having an unoriented distribution and in which a homogenous and uniform space distribution of the fibrous material cannot take place.
Such porous preforms can be infiltrated by a molten metal, for instance aluminum, giving rise to finished manufactures or to semifinished products, which are subsequently treated by customary metallurgic techniques.
The preforms obtained by the processes known to the prior art, however, are characterized by drawbacks, which make their use problematic for the preparation of metal matrix composite structures.
In particular, the high porosity, together with the lack of chemical bonds amid the porous material, give rise to very poor mechanical properties and to such brittleness as to prevent an easy handiness of said preforms.
Moreover, such brittleness involves considerable problems concerning the environment sanitation owing to the characteristics of the fibrous material the pre-shaped products consist of, mainly when the diameters are below one micrometer.
This drawback concerning the brittleness shows during the infiltration phase as well, as the molten metal can have a partial or total destructive action to the preform thereby the distribution of the fibrous material in the composite is not homogenous.
Therefore, to avoid such further drawbacks, sophisticated and consequently expensive infiltration techniques are required, such as metal spray, vapor deposition, vapor infiltration and so on.
Moreover, the composites obtained from these kinds of preforms are characterized in that they have isotropic mechanical properties, because the fibers or whiskers, acting as reinforcement element, are agglomerated in the preform with random distributions, thereby prevented from being able to prepare composites characterized by anisotropic mechanical properties. These anisotropic properties can be generally achieved by orientation of the reinforcement material at present obtained by subjecting the composite to mechanical actions at a high temperature, such as drawing, calendering and the like.